The present invention relates to an active-matrix liquid crystal display driven by digital signals.
Active-matrix displays are usually driven by analog signals that control drive voltages for liquid crystals, such as, disclosed in Japanese Unexamined Patent Publication No. 11-174410 (1999).
There are several modes for liquid crystals, such as VA (Vertical Aligned) and MTN (Mixed-Mode Twisted Nematic). Particularly, VA is used for achieving high contrast ratio.
Active-matrix displays have multiple pixels formed with a liquid crystal filled between an active-matrix substrate and another substrate facing the former substrate. A signal supplied to each pixel is stored in a capacitor provided for the pixel, to drive the liquid crystal.
This type of active-matrix display provides enhanced gradation with voltages supplied to the liquid crystal constant for one-field period but varying in accordance with the level of video signals. Nevertheless, this type of display is prone to generation of noises on the video signals and effects of pseudo video signals, with D. C. components being easily applied to the liquid crystal to cause residual images, thus shorting the life of a display panel.
Another type of active-matrix display is driven by digital signals converted from analog video signals. Pulse voltages are applied to liquid crystals so that the liquid crystals are turned on or off per subfield of several subfields into which one field (one TV field) is divided. Known driving techniques are, such as, using weighted subfields, intra-field dispersion and CLEAR (Hi-Contrast and Low Energy Address and Reduction of False Contour Sequence), such as, disclosed in Japanese Unexamined Patent Publication No. 2001-343950.
This type of active-matrix display is driven by, for example, 8-bit digital signals converted from analog video signals based on CRT reverse-gamma characteristics. In detail, the analog video signals are converted into digital signals based on data stored in a look-up table for weighting corresponding to the gamma characteristics to provide correct gradation levels, due to S-shaped output-light intensity v. s. liquid-crystal driving voltage characteristics.
The digital-based drive technique explained above, causes variation in the gamma characteristics, such as, shown in FIG. 1, when the response speed of liquid crystals varies.
Shown in FIG. 1 is temperature dependency of the gamma characteristics per bit of 256 bits (gradation) in which the gamma characteristics (output light ratio) exhibits “1” at 40.7° C. in temperature and 0.08 Pa·s (Pascal second) in viscosity of liquid crystals.
The viscosity and temperature of liquid crystals have a (strong) correlation, for example, the viscosity becomes low as the temperature rises. In FIG. 1, the viscosity varies from 0.15 to 0.05 Pa·s while the temperature varies from 34.1 to 50.4° C.
It is revealed from FIG. 1 that change in output light at gradation levels is larger in the range from 50 to 100 bits, the intermediate gradation in 256 gradation levels (8 bits/monochrome color). The change in output light occurs due to change in response of liquid crystals to input pulses caused by change in physicality of the liquid crystals when the temperature varies. Several factors cause the change in physicality of liquid crystals, such as, reflectivity, dielectric constant, elastic coefficient, and viscosity of the liquid crystals. Among them, the viscosity is the major factor.
The curves shown in FIG. 1 were given by driving the liquid crystals with pulses shorter than the response time of the liquid crystals.
Higher response speed of liquid crystals provides larger output thanks to higher followability to a single driving pulse shorter than the response time of liquid crystals whereas lower output due to faster response of the liquid crystals to a no-voltage application periods between pulses of a plurality of such single driving pulses.
In contrast, lower response speed of liquid crystals provides lower output due to lower followability to a single driving pulse shorter than the response time of liquid crystals whereas higher output due to slower response of the liquid crystals to a no-voltage application periods between pulses of a plurality of such single driving pulses.
The change in response speed of liquid crystals is one of the factors of the variation in the gamma characteristics in the intermediate gradation. The variation in the gamma characteristics cannot be compensated for, only, by increasing or decreasing the output, because it is a non-linear variation, hence no feasible compensation techniques being proposed.